1. Field of the Invention
This invention relates to a semiconductor device, specifically to a PIN photodiode, a bipolar transistor and an integrated circuit (IC) of an electronically active element such as an n-p-n bipolar transistor and PIN photo-diode.
2. Related Background Art
As a photoelectric conversion device, the PIN photodiode is known, and there have been proposed various techniques for its integration with other devices, e.g., a bipolar transistor, on the same substrate. For example, Japanese Patent Laid-Open (JPO) Nos. 62-123783 and 63-93174 relating to K. Ogawa invention discloses the technique for providing a dielectric film between a silicon crystal layer and a silicon substrate for electric isolation. JPO No. 62-158373 relating to K. D. Moyer invention discloses the technique for thinning that portion of a silicon substrate in a region where a silicon photodiode is formed for decreasing the leakage current. A technique 25 equivalent to Moyer technique is disclosed also in JPO No. 62-18075 relating to Y. Hayashi invention. Furthermore, JPO No.63-19882 relating to T. Ito invention discloses the technique for thinning that portion of a silicon substrate in a region where a photodiode is formed and electrically isolating the photodiode and a transistor by a p-n junction with a reverse-bias voltage applied to. JPO No. 62-16568 relating to M. Yamamoto invention discloses the technique for enclosing a photodiode with a dielectric layer for its isolation from other devices, e.g., a transistor.
But, in these techniques, the step of forming an isolation layer is complicated, and adversely high costs are their problem. Another problem is that the isolation layer is thick, and the integration efficiency of devices falls. When the integration efficiency lowers, wiring, such as aluminum, becomes longer which results in a problem that parasitic capacitance increases unsuitably for high-speed operation.
A first object of this invention is to provide a semiconductor device having a photodiode and an electronically active element such as a bipolar transistor integrated monolithically on the same substrate, and also has high-speed performance.
A second object of this invention is to provide a process for producing a bipolar transistor which can be efficiently isolated from other devices.
A third object of this invention is to provide a photodiode having high-speed performance.